1. Field of the Invention
The present invention relates to a method of manufacturing magnetic powder, magnetic powder and bonded magnets. More specifically, the present invention relates to a method of manufacturing magnetic powder, magnetic powder manufactured by the method, and a bonded magnet manufactured using the magnetic powder.
2. Description of the Prior Art
Rare-earth magnetic materials formed from alloys containing rare-earth elements have high magnetic properties. Therefore, when they are used for magnetic materials for motors, for example, the motors can exhibit high performance.
Such magnetic materials are normally manufactured by the quenching method using a melt spinning apparatus, for example. Hereinbelow, a description will be made with regard to the manufacturing method using the melt spinning apparatus.
FIG. 21 is a sectional side view which shows the situation caused at or around a colliding section of a molten alloy with a cooling roll in the conventional melt spinning apparatus which manufactures a magnetic material by means of a single roll method.
As shown in this figure, in the conventional method, a magnetic material of a predetermined alloy composition (hereinafter, referred to as xe2x80x9calloyxe2x80x9d) is melt and such a molten alloy 60 is injected from a nozzle (not shown in the drawing) so as to be collided with a circumferential surface 530 of a cooling roll 500 which is rotating relative to the nozzle in the direction indicated by the arrow A in FIG. 21. The alloy which is collided with the circumferential surface 530 is rapidly cooled down (quenched) to be solidified, thereby producing a ribbon-shaped magnetic material (that is, a melt spun ribbon 80) in a continuous manner. In this regard, it is to be noted that the dotted line in FIG. 21 indicates a solidification interface 710 of the molten alloy 60.
In the method described above, since the rare-earth elements are liable to oxidize and when they are oxidized the magnetic properties thereof tend to be lowered, the manufacturing of the melt spun ribbon 80 is normally carried out under an inert gas atmosphere.
However, this causes the case that gas enters between the circumferential surface 530 and the puddle 70 of the molten alloy 60, which results in formation of dimples (depressions) 9 in the roll contact surface 810 of the melt spun ribbon 80 (that is, the surface of the melt spun ribbon which is in contact with the circumferential surface 530 of the cooling roll 500). This tendency becomes prominent as the peripheral velocity of the cooling roll 500 becomes large, and in such a case the area occupied by thus formed dimples also becomes larger.
In the case where such dimples 9 (especially, huge dimples) are formed, the molten alloy 60 can not sufficiently contact with the circumferential surface 530 of the cooling roll 500 at the locations of the dimples due to the existence of the entered gas, so that the cooling rate is lowered to prevent rapid solidification. As a result, at portions of the melt spun ribbon where such dimples are formed, the crystal grain size of the alloy becomes coarse, which results in lowered magnetic properties.
Magnetic powder obtained by milling such a melt spun ribbon having the portions of the lowered magnetic properties has larger dispersion or variation in its magnetic properties. Therefore, bonded magnets formed from such magnetic powder can have only poor magnetic properties, and corrosion resistance thereof is also lowered.
In view of the above problem involved in the prior art, it is an object of the present invention to provide a method of manufacturing magnetic powder which can provide bonded magnets having excellent magnetic properties and reliability. Further, it is also an object of the present invention to provide magnetic powder and bonded magnets having excellent magnetic properties and reliability.
In order to achieve the above object, the present invention is directed to a method of manufacturing magnetic powder in which the magnetic powder is manufactured by milling a ribbon-shaped magnetic material which has been obtained by colliding a molten alloy of a magnetic material to a circumferential surface of a rotating cooling roll so as to cool and then solidify it. This method is characterized in that the cooling roll is formed with gas flow passages as gas expelling means for expelling gas entered between the circumferential surface and a puddle of the molten alloy in the circumferential surface thereof, and, when the average pitch of these gas flow passages is defined as Pxcexcm and the average particle size of the magnetic powder is defined as Dxcexcm, the relationship represented by the formula P less than D is satisfied.
According to the above described manufacturing method, it is possible to provide magnetic powder from which bonded magnets having excellent magnetic properties and reliability can be manufactured.
In this method, it is preferred that the average particle size of the magnetic powder lies in the range of 5 to 300 xcexcm. This makes it possible to provide bonded magnets having especially excellent magnetic properties.
Further, it is also preferred that the average pitch P of the gas flow passages lies in the range of 0.5 xcexcm or more and less than 100 xcexcm. When such a cooling roll is used, dispersion in the cooling rates of the molten alloy can be made small irrespective of the contacting portions of molten alloy with the cooling roll, and, as a result thereof, it is possible to provide bonded magnets having especially excellent magnetic properties.
Furthermore, it is also preferred that the average width of the gas flow passages lies in the range of 0.5 to 90 xcexcm. When such a cooling roll is used, gas that entered between the puddle of the molten alloy and the circumferential surface of the cooling roll can be effectively expelled through the passages, and, as a result thereof, it is possible to provide bonded magnets having especially excellent magnetic properties.
Moreover, it is also preferred that the average depth of the gas flow passages lies in the range of 0.5 to 20 xcexcm. When such a cooling roll is used, it is also possible to expel gas that entered between the puddle of the molten alloy and the circumferential surface of the cooling roll effectively through the passages, and, as a result thereof, it is possible to provide bonded magnets having especially excellent magnetic properties.
Further, in a preferred form of this method, when the average width of the gas flow passages is defined as L1 and the average depth of the gas flow passages is defined as L2, the relationship represented by the formula of 0.5xe2x89xa6L1/L2xe2x89xa615 is satisfied. Use of such a cooling roll also makes it possible to expel gas that entered between the puddle of the molten alloy and the circumferential surface of the cooling roll effectively through the passages, so that it is possible to provide bonded magnets having especially excellent magnetic properties.
In this method, it is preferred that the cooling roll includes a roll base and an outer surface layer provided on an outer peripheral portion of the roll base, and the gas flow passages are provided in the outer surface layer. Use of such a cooling roll also makes it possible to provide bonded magnets having excellent magnetic properties and reliability.
In this case, it is preferred that the outer surface layer of the cooling roll is formed of a material having heat conductivity lower than the heat conductivity of the structural material of the roll base at or around a room temperature. This makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, thereby enabling to provide bonded magnets having especially excellent magnetic properties.
Further, it is also preferred that the heat conductivity of the outer surface layer of the cooling roll at or around a room temperature is equal to or less than 80 W mxe2x88x921 Kxe2x88x921. This also makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, so that it is possible to provide bonded magnets having especially excellent magnetic properties.
Preferably, the outer surface layer of the cooling roll is formed of a ceramics. This also makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, thereby enabling to provide bonded magnets having especially excellent magnetic properties. Further, the durability of the cooling roll is also improved.
Further, it is preferred that the thickness of the outer surface layer of the cooling roll is 0.5 to 50 xcexcm. This also makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, so that it is possible to provide bonded magnets having especially excellent magnetic properties.
In this method, it is also preferred that the outer surface layer of the cooling roll is manufactured without experience of machining process. Namely, according to the present invention, the surface roughness Ra of the circumferential surface of the cooling roll can be made small without machining process such as grinding or polishing.
Further, in this method, it is also preferred that the angle defined by the longitudinal direction of the gas flow passages and the rotational direction of the cooling roll is equal to or less than 30 degrees. This also makes it possible to effectively expel the gas that has entered between the puddle and the circumferential surf ace of the cooling roll, so that it becomes possible to manufacture bonded magnets having especially excellent magnetic properties.
Furthermore, it is also preferred that the gas flow passages are formed spirally with respect to the rotation axis of the cooling roll. According to such a structure, it is possible to form the cooling roll with the recesses relatively easily. Further, this also makes it possible to effectively expel the gas that has entered between the puddle and the circumferential surface of the cooling roll, so that it becomes possible to provide bonded magnets having especially excellent magnetic properties.
Moreover, it is also preferred that each gas flow passage has openings located at the peripheral edges of the circumferential surface. This makes it possible to effectively prevent the gas that has once expelled from reentering between the puddle and the circumferential surface again, so that it becomes possible to manufacture bonded magnets having especially excellent magnetic properties.
Further, in this method, it is preferred that the ratio of the projected area of the gas flow passages with respect to the projected area of the circumferential surface is in the range of 10-99.5%. This makes it possible to quench the molten alloy of the magnetic material with an appropriate cooling rate, so that it is possible to provide bonded magnets having especially excellent magnetic properties.
Furthermore, in this method, it is also preferred that the shape of the circumferential surface of the cooling roll is transferred to at least a part of the roll contact surface of the ribbon-shaped magnetic material. According to this method, it is possible to obtain magnetic powder which can provide good binding with the binding resin. Namely, it is possible to obtain magnetic powder which is suited for manufacturing bonded magnets having high mechanical strength and excellent magnetic properties and corrosion resistance.
Another aspect of the present invention is directed to magnetic powder which is manufactured according to the manufacturing method as described above. This magnetic powder can provide bonded magnets having excellent magnetic properties and reliability.
In the present invention, it is preferred that the magnetic powder contains particles each of which is formed with a plurality of recesses or ridges in at least a part of its surface. This makes it possible to provide magnetic powder having good binding force with the binding resin. As a result, this magnetic powder is suited for manufacturing bonded magnets having high mechanical strength and excellent magnetic properties and corrosion resistance.
In this case, it is preferred that the average diameter of the particles of the magnetic powder is defined as Dxcexcm, the average length of the ridges or recesses is equal to or greater than D/40 xcexcm. This also makes it possible to provide magnetic powder having good binding force with the binding resin. As a result, this magnetic powder is also suited for manufacturing bonded magnets having high mechanical strength and excellent magnetic properties and corrosion resistance.
Further, it is also preferred that the average height of the ridges or the average depth of the recesses is in the range of 0.1 to 10 xcexcm. This also makes it possible to provide magnetic powder having good binding force with the binding resin. As a result, this magnetic powder is also suited for manufacturing bonded magnets having high mechanical strength and especially excellent magnetic properties and corrosion resistance.
Furthermore, it is also preferred that the ridges or recesses are formed in parallel with each other, in which the average pitch of the adjacent ridges or recesses is in the range of 0.5 to 100 xcexcm. This also makes it possible to provide magnetic powder having good binding force with the binding resin. As a result, this magnetic powder is also suited for manufacturing bonded magnets having high mechanical strength and especially excellent magnetic properties and corrosion resistance.
Moreover, it is also preferred that the ratio of an area of a portion of the particle where the ridges or recesses are formed with respect to the total surface area of the particle is equal to or greater than 15%. This also makes it possible to provide magnetic powder having good binding force with the binding resin. As a result, this magnetic powder is also suited for manufacturing bonded magnets having high mechanical strength and especially excellent magnetic properties and corrosion resistance.
In the magnetic powder of the present invention, it is preferred that the average particle size of the magnetic powder is in the range of 5 to 300 xcexcm. Use of the magnetic powder containing such particles makes it possible to provide bonded magnets having more excellent magnetic properties.
Further, in the magnetic powder of the present invention, it is also preferred that the magnetic powder is subjected to at least one heat treatment during or after the manufacturing process thereof. This also makes it possible to provide bonded magnets having more excellent magnetic properties.
Preferably, the magnetic powder of the present invention has a composite structure composed of a hard magnetic phase and a soft magnetic phase. This makes it possible to provide magnets having especially excellent magnetic properties.
In this case, it is preferred that the average crystal grain size of each of the hard magnetic phase and the soft magnetic phase is in the range of 1-100 nm. This also makes it possible to provide magnets having excellent magnetic properties, especially excellent coercive force and rectangularity.
Other aspect of the present invention is directed to a bonded magnet which is manufactured by binding the magnetic powder as described above with a binding resin. These bonded magnets have excellent magnetic properties and reliability.
Further, yet other aspect of the present invention is also directed to a bonded magnet which is manufactured by binding the magnetic powder described above with a binding resin, wherein the binding resin enters between the ridges or into the recesses. These bonded magnets have more excellent magnetic properties and reliability.
Preferably, the bonded magnet is manufactured by a warm molding. By using this molding method, the magnetic powder can be bonded with the binding resin more reliably. As a result, it is possible to easily provide bonded magnets having low void ratio and having especially excellent mechanical strength, magnetic properties and corrosion resistance.
In the bonded magnet of the present invention, it is preferred that the intrinsic coercive force (HCJ) of the bonded magnet at a room temperature lies within the range of 320-1200 kA/m. This makes it possible to provide bonded magnets having excellent heat resistance and magnetizability as well as sufficient magnetic flux density.
Further, it is also preferred that the maximum magnetic energy product (BH)max of the bonded magnet is equal to or greater than 40 kJ/m3. By using such a bonded magnet, it is possible to provide high performance small size motors.
Furthermore, it is also preferred that the content of the magnetic powered contained in the bonded magnet is in the range of 75 to 99.5 wt %. The bonded magnets containing the magnetic powder of this amount can have especially excellent mechanical strength, magnetic properties and corrosion resistance.
Moreover, it is also preferred that the mechanical strength of the bonded magnet which is measured by the shear strength by punching-out test is equal to or greater than 50 MPa. This bonded magnet can have especially excellent mechanical strength.
These and other objects, structures and advantages of the present invention will be apparent from the following detailed description of the invention and the examples taken in conjunction with the appended drawings.